Cytochrome P450 enzymes (CYPs) are major drug metabolizing enzymes and experimental pharmaceutical compounds are generally evaluated for their CYP induction potential early in the development process. Measurement of a CYP induction profile in response to a chemical can be used as a fundamental aspect of drug safety evaluation, but expression of these proteins is regulated by transcriptional, post transcriptional and translational mechanisms. As a result, mRNA-based assays are not reliable predictors of CYP induction. Further, CYPs exhibit extensive amino acid sequence homology, particularly within subfamilies, so that P450 protein expression methods are also poorly discriminatory.